Reviews - Dental and Medical Problems

Reviews
Dent. Med. Probl. 2012, 49, 3, 433–437
ISSN 1644-387X
© Copyright by Wroclaw Medical University
and Polish Dental Society
Karol Nosalik1, A, B, D, F, Maciej Kawala2, A, B, F
Contemporary NiTi Archwires – Mechanical Properties
Współczesne łuki NiTi – właściwości mechaniczne
1
2
Orthodontics Clinic, 5th Military Hospital with polyclinic in Cracow, Poland
Division of Dental Prothetics, Wroclaw Medical University, Poland
A – koncepcja i projekt badania; B – gromadzenie i/lub zestawianie danych; C – opracowanie statystyczne;
D – interpretacja danych; E – przygotowanie tekstu; F – zebranie piśmiennictwa
Abstract
Nowadays, in times of strong commercialization of the medical sector, the conscious selection of orthodontic archwires based on scientific evidence has a crucial meaning. The unique mechanical properties of NiTi alloy make it
one of the most commonly used materials for orthodontic archwires. This article defines terms specific to a group
of NiTi archwires, such as thermoelastic martensitic transformation, temperature transitional range (TTR) of martensitic transformation, shape memory effect, superelasticity and the associated termoelasticity and pseudoelasticity. This article aims to systematize the knowledge of the mechanics of NiTi archwires available today based on the
literature review (Dent. Med. Probl. 2012, 49, 3, 433–437).
Key words: NiTi archwires, superelasticity, mechanical properties.
Streszczenie
Obecnie, w czasach silnej komercjalizacji sektora medycznego, świadomy dobór leczniczych łuków ortodontycznych na podstawie dowodów naukowych nabiera zasadniczego znaczenia. Niezastąpione właściwości mechaniczne
stopu NiTi spowodowały, że jest to jeden z najczęściej stosowanych materiałów do produkcji łuków ortodontycznych. W pracy zdefiniowano terminy swoiste dla grupy łuków NiTi, takie jak: termosprężysta przemiana martenzytyczna, zakres temperatur przemiany martenzytycznej (TTR), zjawisko pamięci kształtu, superelastyczność
oraz związana z nią termoelastyczność i pseudoelastyczność. Celem pracy jest usystematyzowanie wiedzy z zakresu
mechaniki dostępnych współcześnie łuków NiTi na podstawie przeglądu piśmiennictwa (Dent. Med. Probl. 2012,
49, 3, 433–437).
Słowa kluczowe: łuki NiTi, superelastyczność, właściwości mechaniczne.
Nowadays, particular emphasis is placed on
evidence-based medicine; nevertheless, conscious
selection of orthodontic archwires based on clinical trials is of paramount importance. However,
the intensive commercialization of the medical
sector, manifested by persistent marketing of companies which offer archwires, very often stands in
opposition to that trend. Sellers’ assurances of the
‘magical’ effects of the improved archwires are not
always supported by specific information regarding temperature transitional ranges of martensite
transformation (TTR) of NiTi archwires, mechanical properties or conditions in which the trials
were performed. Frequently, doctors are misled by
the name of the material which suggests the presence of superelastic properties. For the assessment
of clinical applications of NiTi archwires, two fundamental attributes are of great significance: temperature transitional range (TTR), which ought
to be similar to the temperature of the oral cavity and low deactivation force affecting the periodontium [1, 2].
Contemporary NiTi archwires comprise a big
group of materials varying in properties. They are
extensively used in the orthodontic practice, particularly at the initial stage, due to their high elasticity and wide working range. Proffit presents
a general division of NiTi archwires into two ma-
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K. Nosalik, M. Kawala
jor categories: M-NiTi (stabilized martensitic alloys) and A-NiTi (austenitic alloys). M-NiTi wires
are much springier than other orthodontic alloys
(stainless steel, cobalt-chromium, beta-titanium)
and are quite strong, but they have poor formability and are not described as having shape memory effect or superelastic behaviour. The A-NiTi
group includes archwires utilizing the shape memory effect and superelasticity that is manifestetd by
very large reversible deformations [3]. Discrepancies between the mechanical properties of the two
groups in question resulted in their different applications. Thin, round A-NiTi archwires are used
when a wide range of activation, generating small,
constant force is required. Thicker, and very often
more rectangular M-NiTi archwires are employed
in later stages of treatment when some springy but
stiffer material is desired [3].
In order to assess the clinical applicability of
orthodontic archwires three main properties are
essential: strength, stiffness or springiness and
working range. In practice, higher springiness allows using higher activation of the archwire [3–8].
All of the properties may be defined by referring
to the load-deflection plot [2]. The diagram is
a graphical representation of the wire’s behaviour
while being subjected to bending.
For M-NiTi archwires the load-deflection plot
bears some similarity to other orthodontic alloys (steel, TMA) [9]. Initially, the curve runs in
a straight line. Its inclination in relation to the YAxis represents the stiffness of the wire. The smaller the inclination the more springiness there is in
the archwire [3, 9, 10].
The load-deflection plot for A-NiTi archwires
differs considerably (Fig. 1). Its characteristic fea-
ture is so-called activation/deactivation plateau.
The load-deflection curve, after exceeding a certain force value becomes a horizontal line (activation plateau) and the archwire may absorb more
load, generating a deactivation force (deactivation plateau), on the periodontal fibres, the value
of which is constant and lower than the value of
the activation force. The phenomenon of the discrepancy between the activation and deactivation
force is referred to as the elastic hysteresis [2]. The
hysteresis loop for the superelastic Chinese NiTi is
characterized by being flat while loading and unloading [11]. Putting it into practice, the superelastic NiTi archwire exerts a smaller force upon the
periodontal fibres than that which was used to put
the archwire into the bracket slot (Fig. 2). Lower deactivation forces exerted by the superelastic
nickel-titanium archwires correspond to the physiological response of the bone and minimize the
negative undermining resorption [12].
After Begg’s introduction of the low force technique, a search was initiated for a material with
lower stiffness and increased elasticity to replace
the known and highly valued stainless steel [9].
The groundbreaking, though accidental, discovery was made by Buhler et al. in 1963. They observed that an object treated with heat which was
earlier deformed, returns to its original shape in
a particular temperature range. The return to the
shape before the deformation is referred to as the
Shape Memory Effect (SME) [4, 9, 13–15]. Thanks
to Andreasen’s commitment it was introduced to
orthodontics in 1972. The first orthodontic NiTi
was a martensitic stabilized alloy and did not show
the shape memory effect in the intraoral temperature range. It was characterized by a linear load-
Fig. 1. Diagram representing the load-deflection curve
for stainless steel, TMA and NiTi alloy
Fig. 2. Diagram representing the load-deflection curve
for superelastic NiTi alloys
Ryc. 1. Schemat przedstawiający wykres zależności
między naprężeniem a odkształceniem dla stali
nierdzewnej, stopu TMA i stopu niti
Ryc. 2. Schemat przedstawiający wykres zależności
między naprężeniem a odkształceniem dla superelastycznych stopów niti
Contemporary NiTi Archwires
-deflection curve and was much more elastic than
steel. A main drawback of the first NiTi was the
lack of clinically applicable formability (impossible to bend) [4, 9]. Trials were continued, which
resulted in the introduction two new superelastic alloys – Chinese and Japanese NiTi in the nid
– 1980 s. Further research led to selecting active
NiTi alloys with an active martensite structure
(thermoelastic archwires) and the active austenite
(pseudoelastic archwires) [9, 16, 17].
The characterization of NiTi archwires requires employing notions specific to this type of
materials. They include: thermoelastic martensite
transformation, temperature transitional range,
shape memory effect, superelasticity and related
thermoelasticity and pseudoelasticity. Contemporary NiTi archwires can be found in two crystal structures: low-temperature form – martensite and high-temperature austenitic form. Going
from one phase to the other is called thermoelastic martensite transformation, which is a reversible phenomenon and is dependent on the changes
of the temperature of the environment or the applied force. The transformation is characterized by
the occurrence of the transitory phase R between
martensite and austenite and the specific temperature range for this transformation (TTR), the beginning and end of phase forming: Ms, Mf, As and
Af. The martensite transformation is of great importance for the clinical use of NiTi archwires as it
is responsible for the Shape Memory Effect and the
pseudoelastic properties (SIM) [1, 15, 18].
In the NiTi alloys group the temperature transitional range is broad and it ranges from –200oC
to +200oC. A subtle change of chemical composition or thermo-mechanical processing has a great
influence upon the TTR value [15, 19]. The addition of copper in CuNiTi alloys results in the narrowing of the TTR e.g. for Thermo-Active CopperNiTi40 ® the transformation from martensite to
austenite starts at 24.5oC and ends at 40oC [30]. The
forces generated by this archwire are low; therefore, it is recommended for patient experiencing
periodontal problems. In practical terms, TTR is
of vital importance when assessing the quality of
the archwire, since it is responsible for the presence of superelastic properties. TTR trials indicated that the Af temperature of half of the archwires in question varied from 28oC to 38oC, which
proves that in the intraoral environment they are
austenitic archwires. Applying the archwires, the
Af of which is significantly higher than the body
temperature (Nitinol® – 62oC), does not result
in any force or exerting intermittent or irregular force on the periodontal fibres [1, 11, 20, 30].
From the clinical point of view, TTR should oscillate between slightly below or close to the temper-
435
ature in the oral cavity i.e. from 35oC to 37oC, so
that the archwire can present phase R, the transition between martensite to austenite. As a general
rule, it must be acknowledged that the superelastic archwire in the austenite phase is stiffer than
in the martensite phase, however both these phases are stiffer than the same archwire in the transitory phase R [1].
Thermoelastic archwires producers claim that
between 37–40oC, archwires are fully active, assuming 37oC is the dominant intraoral temperature. The range of temperature in the oral cavity, however, varies from one area to another and
depends on the food eaten and the patient’s behaviour – habitual breathing through the mouth,
smoking cigarettes, and is on average 35.5oC.
Thermoactive archwires, the Af of which is close
to 40oC (40°C Thermo-Active Copper NiTi, NeoSentalloy® 200 g) should not be used at the initial
stages with the patients breathing through their
mouths, whose intraoral temperature ranges 30–
33oC [21, 30].
As far as the biomechanics is concerned, an
important phenomenon occurring within the NiTi archwires group is the regaining of shape after deformation. The shape memory effect occurs
as a result of thermoelastic martensite transformation, i.e. the transfiguration of crystallographic structure between the martensite and austenite within a specific TTR [23]. In the active alloys
(thermoelastic and pseudoelastic) the effect of regaining the shape is a consequence of a mechanical action (force reduction) or temperature action
(heating) [4, 19, 22]. In terms of the clinical use, if
TTR is too similar to the temperature in the oral
cavity, the shape memory effect may occur whilst
placing the archwire and consequently hamper
a precise adjustment of the archwire in the bracket slot. When TTR is above the intraoral temperature, achieving the shape memory effect is possible
by rinsing the oral cavity with warm liquids [19].
NiTi archwires can easily transform their crystalline structure from martensite into austenite
when subjected to the temperature change – thermoelasticity, or from austenite to martensite as
a result of exerting force – pseudoelasticity [34].
The presence of these two properties proves the
superelasticity of the archwire [12]. The result of
the phenomenon of thermoelasticity is the regaining of the shape – SME. In practical terms, a patient may activate or deactivate the archwire by following the doctor’s instructions and washing his
mouth with either cool or warm liquids. The scientific base enabling the use of thermoelastic archwires are various accounts arguing that bone remodels itself more effectively if subjected to a dynamic load rather than a static one [26, 36, 38].
436
K. Nosalik, M. Kawala
Pseudoelasticity occurs locally in a place of large
deformation, forming a phase of the stress inducted martensite – SIM, in the archwire whose structure is almost entirely austenitic. So as to take advantage of the pseudoelastic properties, the deformation of the NiTi archwire above 50–70 degrees
is necessary. Only with such a bend, the superelastic plateau of activation can be achieved. From the
clinical perspective, this situation can be encountered most frequently in the vicinity of the incisors
of the lower jaw, where teeth are crowded and the
distance between brackets are reduced. The trials
confirm a higher efficiency of superelastic multistranded arches than superelastic single-core in
the phase of levelling [24, 31]. The use of rectangular superelastic archwires (e.g. CuNiTi) in the
initial phase of levelling is supported by the idea
that they generate low, but constant force retaining
the three-dimensional control of the tooth’s position. Nevertheless, as some studies prove, the torsion of superelastic rectangular archwire does not
generate superelastic behaviour [30, 33]. There are
no significant differences in functioning between
CuNiTi archwires and superelastic NiTi archwires
in the phase of levelling [32]. Some of the archwires
exhibit superelastic properties, but only if the bend
is larger than 2 mm [30]. The size of the superelastic archwire is of no great influence over the generated force. For NiTi of 0,018” diameter as well
as for 0,020”, the plateau of deactivation was displayed on the same level [35]. Considerably higher
efficiency of superelastic archwires is present only
in the application employing Begg’s slot. Their effectiveness compounded with the use of the edgewise-type appliance is comparable to the multistranded steel archwires [37]. If the crowding is
moderate, superelastic archwires need not to be
used. In that case, martensitic stabilized archwires
(nitinol) of small diameter or multistranded steel
wire are sufficient. For achieving optimal forces, it
is essential to select an appropriate diameter and to
eliminate rectangular archwires from the levelling
phase [2]. Trials evaluating superelastic and conventional NiTi archwires with regard to the movement speed of a tooth as well as the pain sensation
have not validated the differences in their properties [2]. The superelastic A-NiTi significantly reduces the need for replacing thin archwires with
the thicker for levelling and alignment [19].
There are many classifications of NiTi archwires. Kusy categorizes them taking the crystalline
structure into consideration as follows: conventional nitinol-stabilized martensite, Pseudoelastic-active austenite and Thermoelastic-active martensite [4]. Waters provides a different classification, based on the dissimilarities in TTR: I – active
martensite alloys the TTR of which ranges from
the room temperature to the body temperature, II
– austenite alloys whose TTR is below the room
temperature, III – alloys characterized by narrow
TTR close to the body temperature [27]. There is
also a dichotomy splitting them into superelastic
and tempered NiTi [25]. Evans and Durning introduced a classification of orthodontic alloys into five categories: 1) stainless steel, gold; 2) first
stabilized NiTi; 3) superelastic wires – active austenites TTR of which is below the intraoral temperature (pseudoelastic); 4) thermodynamic wires
– active martensite whose TTR is close to the intraoral temperature; 5) a group of thermodynamic wires featuring diverse TTR (e.g. cuNiTi 35oC,
37oC, 40oC) [28, 29].
To conclude, NiTi archwires must be regarded as an indispensible tool in the orthodontist’s
inventory. The selection of NiTi archwires ought
to be based on scientific evidence rather than advertisement. Specific information regarding their
mechanical properties is vital while making the
choice of the archwire, as it translates into their
clinical action.
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Address for correspondence:
Karol Nosalik
ul. Sienkiewicza 9/6
30-033 Kraków
Poland
Tel.: 605 367 633
E-mail: [email protected]
Received: 23.06.2012
Revised: 6.07.2012
Accepted: 16.07.2012
Praca wpłynęła do Redakcji: 23.06.2012 r.
Po recenzji: 6.07.2012 r.
Zaakceptowano do druku: 16.07.2012 r.